Combined satellite and terrestrial communication systems are known as Complementary Ground Component (CGC) or Ancillary Terrestrial Component (ATC) systems. In such systems known in the art, the satellite and terrestrial systems generally operate within the same frequency spectrum. The terrestrial system may act as a ‘repeater’ to relay signals or channels also transmitted by the satellite systems to areas where satellite coverage is poor, such as built-up areas. Various operators have been licensed to operate such systems, but their implementation has been hampered by technical problems.
Sharing frequency spectrum between satellite and terrestrial systems gives rise to interference problems. One known technique for mitigating interference involves frequency division between the satellite and terrestrial components, such that the two components do not use the same frequencies at the same time in the same areas. This is, however, wasteful of bandwidth.
Another technique involves measuring the received signal strength or interference levels at one or more terminals, and adapting the system dynamically to avoid interference. For example, US-A-2010/0157857 discloses a technique where a terminal determines whether to communicate with the satellite or CGC systems depending on the signal strength received from the satellite system.